Lithium-ion secondary batteries have high charged and discharged capacities, and are secondary batteries being able to make the outputs high. Currently, the lithium-ion secondary batteries have been used mainly as power sources for portable electronic appliances, and have further been expected as power sources for electric automobiles anticipated to become widespread from now on. The lithium-ion secondary batteries comprise active materials being capable of inserting and eliminating (or sorbing and desorbing) lithium (Li) in the positive electrode and negative electrode, respectively, and lithium ions moving within an electrolytic solution disposed between the two electrodes lead to operating the lithium-ion secondary batteries. A lithium-containing metallic composite oxide, such as lithium/cobalt composite oxides, has been used mainly as an active material of the positive electrode; whereas a carbon material with a multilayered structure has been used mainly as an active material of the negative electrode.
However, since the capacity of the current lithium-ion secondary batteries is not possibly said to be satisfactory, making the current lithium-ion secondary batteries exhibit a much higher capacity has been sought for. As an approach for achieving the target, although making a positive-electrode potential a high voltage has been investigated, the investigation has been associated with such a large problem that, when a battery is driven with a high voltage, especially when a battery is driven at a high temperature with a high voltage, the battery has extremely deteriorated characteristics after being charged and discharged repeatedly. A cause of the problem has been believed to be the oxidation decompositions of an electrolytic solution and electrolyte which occur in the vicinity of a positive electrode when being charged.
That is, the consumption of lithium ions by the oxidation decompositions of an electrolyte in the vicinity of a positive electrode is believed to result in declining a capacity. Moreover, an output is believed to decline because the decomposed products of an electrolytic solution depositing on an electrode surface or in the voids of a separator serve as a resistive matter to act against the lithium-ion conduction. Therefore, to solve such problems, inhibiting the electrolytic solution and electrolyte from decomposing is necessary.
Hence, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 11-097027, Japanese Translation of PCT International Application Publication (KOHYO) Gazette No. 2007-510267, and so on, propose a nonaqueous-electrolyte secondary battery in which a coated layer composed of an ion-conductive polymer, and the like, is formed onto a positive-electrode surface, respectively. The gazettes state that forming the coated layer results in making the following deteriorations inhibitable: the elution of a positive-electrode active material, the decompositions thereof, and so forth.
But, since the gazettes do not at all set forth any evaluation for the nonaqueous-electrolyte secondary batteries when the batteries are charged with such a high voltage as 4.3V or more, the batteries have not been proved clearly yet whether or not being able to endure, such a high-voltage driving mode or operation. Moreover, since the thickness of the coated layers is also on the order of micrometers substantially, the coated layers impede the lithium-ion conduction greatly.